Some years ago I was at a concert with my mother and, come the interval, there was the usual queue for the ladies. As time was running out some off us took over the gents. The men who tried to come in took it for granted that they'd made a mistake and left. Eventually a gentleman in evening dress came in, realised what was happening , and said "I'm sorry ladies but I'm due back on stage in two minutes so you'll just have to turn your backs" which we all did with much hilarity.
I once had an interesting meeting with a Austrian truck driver in a loo at a truck stop in Italy when the ladies side was locked.